Dec. 23, 2004
John Bluck
NASA Ames Research Center, Moffett Field, Calif.
Phone: 650/604-5026
E-mail: jbluck@mail.arc.nasa.gov
RELEASE: 04-115AR
NASA FINDS POLLUTED CLOUDS HOLD LESS MOISTURE AND COOL EARTH LESS
A NASA study found some clouds that form on tiny haze particles are
not cooling the Earth as much as previously thought. These findings
have implications for the ability to predict changes in climate.
Andrew Ackerman, a scientist at NASA Ames Research Center, located in
California's Silicon Valley, and his colleagues found, when the air
over clouds is dry, polluted clouds hold less water and reflect less
solar energy. Ackerman is the study's principal author.
Contrary to expectations, scientists observed polluted, low-lying
clouds do not generally hold more water than cleaner clouds. Low
clouds cool the planet by reflecting sunlight away from the Earth's
surface, and more water makes a cloud more reflective.
Previously, scientific consensus was, that since polluted clouds
precipitate less, they should contain more water and reflect more
sunlight back into space. Most predictions of global climate change
assume less precipitation will result in clouds holding more water,
reflecting more sunlight and counteracting greenhouse warming.
"The natural laboratory we used to look at the contrasts between clean
and polluted clouds is a phenomenon called ship tracks, which are
long lines of clouds with smaller cloud droplets that form on the
exhaust particles from ships," Ackerman said.
"The results of this work should provide for more realistic treatment
of polluted clouds in climate models, improving predictions of future
climate," Ackerman said. "In the meantime, it's critical that we
thoroughly test these new theoretical results. NASA's latest
generation of Earth-observing satellites provides a powerful tool for
doing just that, by observing how ship tracks are affected by the
humidity of the air above them," he said.
Ship track measurements were taken off the west coast of the United
States from polar-orbiting satellites and aircraft flying through the
clouds. The Moderate Resolution Imaging Spectroradiometer (MODIS)
Airborne Simulator instrument (comparable to the MODIS instruments on
NASA's Terra and Aqua satellites), aboard a NASA ER-2 aircraft flying
above the clouds, was also used to gather data. The measurements show
cloud water decreases more often than it increases in polluted
clouds.
To understand how cloud water changes in polluted clouds, the team of
scientists created a 3-D computer model to simulate atmospheric
motions and the formation of precipitation by clouds. The researchers
tested their model by comparing its predictions with measurements
from field projects devoted to characterizing marine stratocumulus
clouds.
After verifying that the model reproduced the behavior of real clouds,
the scientists asked their computer model how pollution affects
clouds. In agreement with previous work, the computer simulations
showed, when air over a cloud is humid, cloud water increases in
polluted clouds. However, when air over a stratocumulus cloud deck is
dry, surprisingly, simulations indicated that water decreased in
polluted clouds, consistent with the behavior observed in ship
tracks.
Ackerman's co-investigators included Michael Kirkpatrick, University
of Tasmania, Hobart, Australia; David Stevens, Lawrence Livermore
National Laboratory, Livermore, Calif.; and O. Brian Toon, University
of Colorado, Boulder. The researchers' findings appear in today's
issue of the journal Nature.
To access a related publication-ready ship track image on the Web,
visit:
http://visibleearth.nasa.gov/cgi-bin/viewrecord?25264
For information about NASA and agency programs on the Internet, visit:
http://www.nasa.gov
For information about the NASA Earth Science Division, visit:
http://geo.arc.nasa.gov/
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